The day after the ACL reconstruction (which is performed with the same technique for all the subjects enrolled), the patients were randomized to receive a single injection of HA or 3 ml of saline solution. The randomization list was kept in a dedicated office and the injecting physician contacted this office to know the treatment allocation just before the injection, that was performed by a lateral parapatellar approach, after removing the drainages and after attempting aspiration to evacuate any eventual residual blood from the joint. In order to maintain the patient blind to the treatment, a surgical drape was placed to prevent him to see the injective procedure.